The invention relates in general to an apparatus for protecting fluid conveying devices from freezing. More particularly, the invention relates to a drain door for facilitating the removal of discharged fluid from an apparatus that protects backflow prevention devices and their associated piping. The invention also relates to an improved locking mechanism, heater and a variety of structural enhancements that aide in the construction and installation of the apparatus of the invention.
Backflow prevention devices are well known and typically include an assembly of one or more check valves installed in a pipeline. These devices prevent the reverse flow, or backflow, of fluid from the downstream pipeline to the upstream supply pipeline or main, which can result when the fluid pressure in the upstream supply pipeline falls below the fluid pressure in the downstream pipeline. The prevention of backflow is particularly important when the downstream pipeline contains contaminated material and the upstream supply pipeline is carrying potable water.
Backflow prevention devices may or may not discharge fluid to the atmosphere. For example, a double check assembly does not discharge fluid. However, when the prevention of backflow is necessitated by health considerations, such as in an installation where contaminated water could be conveyed back into a potable water main, a reduced pressure zone device is included in the assembly. Reduced pressure zone devices discharge a large volume of water when actuated, and thus the downstream water, which may be contaminated, is transferred out of the downstream pipeline to the atmosphere and not back into the upstream supply pipeline. Besides discharging large volumes, these reduced pressure zone devices intermittently discharge small quantities of water. Since these backflow prevention devices are frequently installed in a water supply line outdoors and above ground, it is important that the devices be enclosed and protected from the weather, and particularly from freezing.
It is well known that water lines need to be protected from freezing. Many pipelines are run underground to avoid exposure to ambient temperatures that are below the freezing point of water. Pipelines are also run underground for aesthetic reasons. While a majority of a pipeline may be underground, backflow prevention devices and their associated valving and piping cannot be buried underground because access is needed for proper operation and maintenance of the backflow prevention device. Additionally, discharging backflow prevention devices must be elevated because they must not be submerged in the discharged material. Therefore reduced pressure zone devices cannot be buried in the ground or installed in a floodable pit for the additional reason that they must not be submerged. Thus, in many installations the pipeline is run underground and the portions of the pipeline having backflow prevention devices are constructed above ground level. The backflow prevention devices, valves, and portions of the pipeline are subject to freezing and may become inoperable unless they are protected from the weather.
Years ago, large concrete or cast iron boxes were constructed around backflow prevention devices. A door was provided in the top of the box to provide access to the interior of the box for servicing the device. Open drain holes were formed in the lower portions of the walls of the box to drain any water discharged by the backflow prevention device away from the box. These boxes were inferior due to the material of construction and because access to the interior of the box was difficult. Moreover, due to the open fixed drain holes in these boxes, water, foreign objects and most importantly cold air would enter the interior portion of the box. The movement of cold air into the box defeated a central purpose of its design by facilitating the freezing of the water within backflow prevention device and its associated piping and valving.
U.S. Pat. No. Re 33,523, to Devine, discloses another type of apparatus for protecting a backflow prevention device. This apparatus includes a sectionalized cover which is held together by clamps. The top portion of the cover has openings to permit manipulation of the protruding valve stems which extend above the cover. Drain openings are provided at ground level to permit drainage of liquid from the space within the cover. These drain openings are covered by a screen and vertically cut flaps which hang from the top of the opening to ground level. The cover is insulated and includes an electric heating element placed in the upper portion of the cover interior. Devine does not describe how his apparatus sections are aligned and mounted. Although there are many drawbacks associated with the Devine cover, the openings in the top portion and the flap-covered drain openings allow air to be transferred to and from the interior of the cover, thus significantly increasing the heating requirements of the cover and, in some cases, making freeze protection impractical.
Another type of cover is disclosed in U.S. Pat. No. 4,890,638 (the "'638 patent"), which includes a cover member having a top, which is a unitary structure, and four walls. The '638 patent has drain openings at the bottom portion of its walls to allow water discharged by the backflow prevention device to escape from the interior of the cover. These drain openings are covered by drain doors, which are spring biased to be maintained in a closed position. The '638 patent does not describe a heater or the alignment and mounting structures of the present invention. Additionally, this cover suffers from a variety of drawbacks including the fact that the spring abruptly snaps shut and must be maintained, because it breaks and becomes stretched out after a heavy discharge of fluid through the drain opening.
U.S. Pat. No. 5,609,784 describes an apparatus and method for covering and heating fluid flow devices, which adds a heater positioned on the ground surface in the interior of the cover described in the '638 patent.
The foregoing demonstrates that there is a need for an energy-efficient enclosure which protects backflow prevention devices, improves drain door operation, improves mounting and alignment of the enclosure, increases the efficiency of the heating operation, and facilitates inspection of the backflow prevention device.